The present invention relates to an extrusion head for foamed material, in particular for polyvinylchloride-based foamed material.
Extrusion heads for foamed material, for example PVC with the addition of substances which cause its foaming, are known which substantially comprise a head body in which a cavity accommodating a male element is defined. A passage for the material to be extruded is defined between the outer surface of the male element and the walls of said cavity, and said material is pushed into the extrusion head at a preset pressure. The passage along which the semi-liquid or pasty material is pushed extends around the male element and leads outward through an extrusion hole which is located proximate to a longitudinal end of the head and is shaped correspondingly to the profile of the extruded body to be obtained.
The material pushed through the extrusion head completes its expansion in the region of the extrusion hole, and the extruded body thus obtained follows a rectilinear path, passing through "gauges" arranged in series. Said gauges are substantially constituted by cooled blocks in which a passage is defined with a configuration which reproduces that of the profile of the extruded body. In passing through said gauges, the extruded body is cooled progressively so as to gradually stabilize its configuration.
In many cases, the shaped passage defined in the gauges is kept at negative pressure so as to obtain the adhesion of the extruded body to the walls of said passage, utilizing a residual expansion of the material which allows to obtain extruded bodies with excellent surface finish.
Means for entraining the extruded body are provided downstream of the gauges along the direction of extrusion and are generally constituted by a pair of opposite closed-loop belts or tracks between which the extruded body is inserted so as to advance it in accordance with the extrusion speed.
At the beginning of the extrusion, the initial end of the extruded body is manually gripped and inserted between the pair of tracks, placing the extruded body in the gauges, which are conveniently open during this operation, thus starting the extrusion process which, after the beginning of the entrainment effected by the tracks, can continue automatically with no further manual intervention.
This necessary extrusion-starting operation has some practical problems in execution.
In conventional extrusion heads, the extruded body which has just exited from the extrusion hole is in fact very fragile due to the fact that the material completes its expansion in the air at the exit of the extrusion hole, and this makes its handling troublesome. Due to this fragility, and also due to the fact that since the extruded body is not yet subject to the traction action of the entrainment means it tends to expand more than the passages defined in the gauges, and it is extremely difficult to place the extruded body correctly in the cooling gauges.
The difficulties in handling the extruded body in this initial extrusion step are unavoidably the source of time losses which considerably affect the productivity of an extrusion system.
The initial portion of the extruded body, i.e. a portion thereof which is substantially equal in length to the distance between the extrusion head and the entrainment tracks, must furthermore necessarily be discarded due to the deformations or breakages which occur during this operation.